In the field of graphic arts, a printing plate is produced using a set of color separations of a color original which are prepared using lithographic films. In general, color proofs are prepared from color separations in order to inspect for errors in color separation and to check the need for color correction and the like before printing. Color proofs are required to realize high resolution enabling accurate half tone reproduction and high processing stability. To obtain color proofs close to actual prints, it is desirable for the materials of color proofs to be the same as those used on press, i.e., the same paper and the same pigments. There is a higher demand for a dry process involving no processing solutions for the preparation of color proofs.
With the recent spread of computerized systems in prepress work, recording systems for preparing color proofs directly from digital signals (dry process) have been developed. Such computerized systems, particularly contemplated for preparing high quality color proofs, are generally capable of reproducing dot images at 150 lines or more per inch. In order to obtain high quality proofs from digital signals, a laser beam is used as a recording head, which is capable of modulation according to digital signals and focusing into a small spot diameter. Hence it is demanded to develop image forming elements that exhibit high sensitivity to laser light and high resolution enabling reproduction of highly precise dot images.
Image forming elements known useful in laser transfer methods include a thermal melt transfer sheet, which comprises a substrate, a light-heat conversion layer capable of absorbing laser light to generate heat, and an image forming layer having a pigment dispersed in a heat fusible matrix (e.g., a wax or a binder) in the order described, as disclosed in JP-A-5-58045. A thermal transfer sheet of this type is brought into contact with an image receiving sheet and imagewise irradiated with a laser beam. The irradiated area of the light-heat conversion layer generates heat to melt the image forming layer, and the molten part of the image forming layer is transferred to the image receiving sheet.
JP-A-6-219052 teaches a thermal transfer sheet comprising a substrate, a light-heat conversion layer containing a light-heat converting substance, a release layer as thin as 0.03 to 0.3 μm, and an image forming layer containing a colorant. According to this technique, the release layer reduces its bonding strength between the image forming layer and the light-heat conversion layer upon being irradiated with laser light. As a result, the image forming layer is allowed to be transferred to an image receiving sheet that has been brought into contact with the thermal transfer sheet to form a high precision transfer image. This image formation method utilizes laser ablation. That is, a laser-irradiated part of the release layer decomposes and vaporizes, resulting in reduction of the strength bonding the image forming layer and the light-heat conversion layer in that area. As a result, the image forming layer of that area is transferred to the image receiving sheet.
These imaging methods are advantageous in that images can be formed on printing paper having an image receiving layer (adhesive layer) and that a multicolor image can easily be obtained by successively transferring images of different colors onto the same image receiving sheet. The method utilizing ablation is particularly advantageous for ease of forming a highly precise image and is useful to prepare color proofs (DDCPs) or precise mask images.
With the spread of desk-top publishing (DPT) work, printing companies adopting a computer-to-plate (CTP) system have a strong demand for a DDCP system, which eliminates the need of intermediate film or plate output as has been involved in traditional analog proofing. In recent years, DDCPs with higher quality, higher stability, and larger sizes have been demanded as good approximations to the final prints.
Laser thermal transfer systems are capable of image formation at high resolution. Options include laser sublimation, laser ablation, and laser melt, each of which has the problem that the recorded dot shape is not sharp enough. The laser sublimation system is insufficient in approximation in color to the final print results because of use of dyes as coloring matter. Besides, this system involving dye sublimation results in blurred dot outlines, failing to achieve sufficiently high resolution. The laser ablation system, which uses pigments as coloring matter, provides a satisfactory approximation in color to the final printed products, but the dots are blurred, resulting in insufficient resolution similarly to the dye sublimation system because of the involvement of coloring matter scattering. The laser melt system also fails to create clear dot outlines because the molten colorant flows.
Where a heat transfer sheet the image forming layer of which has a thickness of 0.2 μm or greater is stored for a long time under severe conditions, a pigment dispersant that is not adsorbed onto pigment particles can migrate to the interface between the image forming layer and the light-heat conversion layer. It would follow that the adhesion between the image forming layer and the light-heat conversion layer is reduced, and non-irradiated area (non-image area) might be transferred to the image receiving layer to cause reduction in resolution.
Since the colors used in conventional heat transfer sheets are process colors (i.e., yellow, magenta, cyan, and black) and their combinations, reproducible hues have been so limited.